drm/i2c: tda998x: prepare for video input configuration

The video-input-port (VIP) is highly configurable. This prepares
current driver to allow to configure VIP configuration, as some
boards connect lcd controller and TDA998x "pin-swapped" and depend
on VIP to swap the pins by register configuration.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Tested-by: Darren Etheridge <detheridge@ti.com>
Tested-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Tested-by: Russell King <rmk_kernel@arm.linux.org.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>